This invention relates to inkjet printers and, in particular, to a technique for improving the alignment of dots printed by an inkjet printhead.
U.S. Pat. No. 5,638,101, entitled High Density Nozzle Array for Inkjet Printhead, by Brian Keefe et al., and U.S. Pat. No. 5,648,806, entitled Stable Substrate Structure for a Wide Swath Nozzle Array in a High Resolution Printer, by Steven Steinfeld et al., are assigned to the present assignee and incorporated herein by reference. These two patents describe examples of an inkjet printer, incorporating an inkjet print cartridge, whose operation may be improved by the present invention. The below description of primitives used in printheads is taken from those two patents.
FIG. 1 is a simplified example of an inkjet printer 10. This will be used to illustrate the problem with prior art printers and, later, will also serve as a printer whose operation has been improved after being modified to incorporate the present invention. Inkjet printer 10 includes an input tray 12 containing sheets of paper 14 which pass through a print zone 15 for being printed upon. The paper 14 is then forwarded to an output tray 16. A moveable carriage 20 holds print cartridges 22, 24, 26, and 28, which respectively hold yellow, magenta, cyan, and black inks. The carriage 20 is moved along a scan axis by a conventional belt and pulley system and slides along a slide rod 30.
Printing signals from an external computer are processed by printer 10 to generate a bit map of the dots to be printed. The bit map is then converted into firing signals for the printhead. The position of the carriage 20 as it traverses back and forth along the scan axis is determined from an optical encoder strip 32, detected by a photoelectric element on carriage 20, to cause the various ink ejection elements on each print cartridge to be selectively fired at the appropriate time during a carriage scan.
FIG. 2 illustrates the printhead portion of a print cartridge, such as print cartridge 22 in FIG. 1, while FIG. 3 is a top-down detailed view of a nozzle plate 34 on the print cartridge. Three hundred nozzles 35 are shown. The primitives P1-P14 (to be described later) are labeled on the nozzle plate 34. The print cartridge 22 has contact pads 36 formed on a TAB circuit which electrically contact electrodes in cartridge 20 for receiving power and ground signals as well as the firing signals for the various ink ejection elements.
FIG. 4 illustrates a portion of the printhead substrate, underneath nozzle plate 34, associated with a single primitive. The printhead substrate is a rectangular piece of silicon having formed on it ink channels 40, ink ejection chambers 42, and heater resistors 44 using photolitographic techniques. The various ink channels 40 and chambers 42 are formed by a barrier layer 45 of photoresist. Ink flows into each chamber 42 via an associated ink channel 40. When current passes through a heater resistor 44, ink is vaporized to cause a droplet of ink to be ejected by an associated nozzle. Each ink channel 40 is designed to reduce cross-talk between the ink chambers 42 when fired.
To further reduce cross-talk, and to simplify the firing electronics and wiring, the heater resistors 44 are divided into primitives. FIG. 4 illustrates a single primitive having 22 heater resistors 44.
FIG. 5 illustrates firing circuitry on the substrate for a single heater resistor 44. To fire resistor 44, an address pulse is provided on address select line 46 to turn on drive transistor 47, and a primitive select pulse is provided on primitive select line 48 to cause a current to flow through resistor 44 sufficient to heat the resistor to a temperature needed to vaporize ink within the ink ejection chamber. Electrostatic discharge protection FETs 50 drain unwanted electrostatic charges, and a pull-down resistor 52 places all unaddressed select lines 46 in an off state.
All heater resistors 44 within a primitive receive the same primitive select signal, but only one of the resistors in a primitive at a time receives an address signal. This is illustrated in FIG. 6 where address signals A1 through A22 are generated in sequence for associated heater resistors 44 within each primitive during a single firing cycle while the printhead is scanning across the medium.
More particularly, the address select lines 46 (FIG. 5) are sequentially energized according to a firing order counter located in the printer from A1 to A22 when printing from left to right and from A22 to A1 when printing from right to left. The print data retrieved from the printer memory causes the print engine to energize any combination of the primitive select lines at the appropriate times during the firing cycle. The primitive select pulses rather than the address select pulses are preferably used to control the resistor current pulse width, as shown in FIG. 7. This is more desirable than using the address select pulses to control the pulse width since terminating an address pulse while the drive transistors 47 (FIG. 5) are conducting high current can cause avalanche breakdown and consequent physical damage to the MOS drive transistors. Accordingly, the address select lines are set before power is applied to the primitive select lines, and, conversely, power is turned off before the address pulse is removed. To provide uniform energy per heater resistor 44, only one resistor is energized at a time per primitive. However, any number of the primitives may be enabled concurrently. Each enabled primitive select pulse thus delivers both power and one of the enable signals to the drive transistor. Each address select line is tied to a corresponding address select line in all the other primitives.
Modern print cartridges may print on the order of 300 or 600 dots per inch (DPI), and the width of a printhead along the direction of the column of nozzles may be xc2xd inch or greater.
Due to various factors, it is extremely difficult to print precisely aligned dots on the medium as the printhead is scanning across the medium.
FIG. 8A illustrates an ideal vertical line 54 of connected dots printed during a single scan of the printhead across the medium. FIG. 8B is an exaggerated example of the actual line 56 printed during a single scan which was intended to convey a vertical line. The primitives from FIG. 3 used to print the line are identified in FIG. 8B. The skewing of the line at an angle with respect to the vertical axis 57 is due to the tilting of the print cartridge within the carriage in combination with the tilting of the printhead substrate with respect to the print cartridge. The wavering of the line 56 is due to a number of factors. One of the factors is the variation in the directionality of the ink droplets ejected from the nozzles. Another factor is that the paper may not be perfectly parallel to the plane of the nozzle plate. Another factor is that the nozzle plate may not be perfectly planar. Another factor is the different parasitic capacitances associated with the primitives.
Also, nozzles are formed in two offset columns, as shown in FIG. 3, to increase the density of dots in the direction perpendicular to the scan direction. To print a solid vertical line, the nozzles in the two columns must be fired so as to print dots which partially overlap on the medium. Thus, if the dots printed by the two columns of nozzles are not aligned precisely, distortion of the vertical line will result.
If the vertical line is made up of dots from different printheads, as would be for a composite color line, a blurring of the line would result by the nozzles in the printheads of the various print cartridges not being aligned with respect to each other.
The dot placement due to printhead misalignment gets worse when the printhead length is increased. Longer printheads enable higher throughputs but the manufacturing processes are not able to ensure the planarity of the nozzle plate necessary to guarantee the print quality requirements.
What is needed is a technique for improving the alignment of dots printed by an inkjet printer.
In the inkjet printer discussed previously, to print a vertical line of dots, all primitives in a single column of nozzles (e.g., all even primitives in FIG. 3) were energized at the same time to fire the various heater resistors in the column.
In one embodiment of the inventive technique, printing instructions are provided to the printhead to print a predetermined pattern on a medium, and the printed pattern is detected by optical sensors in the printer. Based on the detection, a position offset error for each primitive is determined. These errors are used to generate a separate time correction for each of the primitives such that, when the printer is used normally, the firing pulses for nozzles in each primitive will be appropriately delayed or advanced so as to align the dots printed by the primitives. This will correct both the skew and line waver shown in FIG. 8B. This technique is referred to as fractional column correction.
In one embodiment, the pattern printed is a pattern of blocks of dots where each primitive prints a block of dots separated from other blocks. Thus, each block is associated with a known primitive. The distance from a centroid of each block printed by a primitive to a reference block printed by a reference primitive is then easily measured. In this manner, the timing corrections may be calculated for each primitive.
In another embodiment of the inventive technique, printing instructions are provided to the printhead while on the manufacturing line to print a predetermined pattern on a medium, and the printed pattern is detected by optical sensors in the manufacturing line. In one embodiment, based on the detection, the x,y position of the printed matter by each primitive relative to the printed matter by a reference primitive in the printhead is determined. These x,y positions are then stored in a non-volatile memory on the print cartridge itself. These x,y positions will be later used to identify dot misalignments associated with each primitive solely due to factors within the print cartridge itself.
The print cartridge is then installed in the carriage of a printer, likely incurring some undesirable rotation of the print cartridge which would normally skew printed lines. A stored program again causes the printhead to print a predetermined pattern on a medium. This pattern is detected by an optical sensor in the printer to measure the x, y position of printed matter printed by the primitive farthest from the reference primitive relative to printed matter printed by the reference primitive. In an alternative embodiment, any primitive position may be referenced to the reference primitive, and the most suitable primitive may be the one with the most stable position or the one easiest to measure.
Based on a comparison of the x,y position in the memory on the print cartridge and the newly detected x,y position, the rotation angle caused by the print cartridge being installed in the carriage is calculated. This rotation angle is then used to translate the original x,y positions in the print cartridge memory into the actual x,y positions, which take into account misalignments caused by the print cartridge as well as the carriage.
The resulting x,y positions are compared to ideal x,y positions, and a position error associated with each primitive is generated.
These errors are used to generate a separate time delay or advance for each of the primitives such that, when the printer is used normally, the firing pulses for nozzles in each primitive will be appropriately delayed or advanced so as to align the dots printed by the primitives. This will correct both the skew and line waver shown in FIG. 8B.
In another embodiment, some of the positional measurements are made by the manufacturing line sensor and the remainder of the measurements are made by the printer line sensor, with no rotational translation made.
By using test equipment in the manufacturing line, more precise measurements can be taken, as compared with measurements taken by the printer, and the printer can calculate the compensation faster.